1. Field of the Invention
This invention relates to an image forming apparatus such as a laser beam printer, and more particularly to an image forming apparatus such a laser beam printer which is capable of receiving data in a relatively high speed in accordance with a data transmission speed provided from a host system to the laser beam printer through an interface operative in a high speed data transmission mode using parallel port interface with DMA (direct memory access) function.
2. Discussion of the Background
There has been an image apparatus capable of receiving data with DMA function which is transmission without controlling of an interface. The DMA function is the function of transmission data from a designated address to a destination address without controlling by a CPU. In this DMA function, when a parallel port is source and a receiving buffer is destination, a high speed receiving of parallel data can be executed.
On the other hand, there has been standardized a new parallel port interface operative for data transmission between a host system and an image forming apparatus such as a laser beam printer, recently by IEEE (Institute of Electrical and Electronics Engineers). This interface is called IEEE-1284 and includes bi-directional communication mode. In this interface, for request bi-directional communication mode, one byte data as expansion code is sent from a host system to an image forming apparatus for acknowledge setting bi-directional communication mode at a phase of negotiation.
In this system, during DMA transmission, when a bi-directional communication mode is requested, not only transmitted data but also the expansion code is taken into the receiving buffer, as a result, the bi-directional communication mode can not be executed and errors of data transmission are occurred. The reason of these errors of data transmission is that the expansion code data which should not be taken into the receiving buffer is transmitted to the receiving buffer because the DMA transmission is executed without controlling by a CPU. Therefore, a high speed parallel transmission by the DMA function and a bidirectional communication mode by software controlling can not coexist.